The White Space of technology

Wi-Fi and cellular technologies such as 2G, 3G and 4G are global standards, but lack of local ubiquity currently rules out Wi-Fi for many an M2M application, and cost rules out cellular. Cellular is also hindered by battery sapping wireless technology. Bluetooth and Zigbee, on the other hand, can operate at very low power, but they lack the range of cellular and Wi-Fi.

Weightless, however, was designed specifically for M2M communications. Its trick: make use of the White Space spectrum. This is now available for unlicensed use in the US, and UK wireless regulator Ofcom may make the spectrum available to Brits by the end of the year. White Space is currently allocated to TV broadcasters, but different parts of the band are used in different locations, leaving space free for other uses.

Complicated rules govern unlicensed users hopping into the band to make sure that incumbents’ licensed toes are not stepped on. Essentially, that puts the onus on the arrivistes to find out what parts of the band are available to them at a given spot, to check this is so and, if necessary, to move at any moment.

The rules also mean they must operate on very low transmission power levels - handy for battery life - such as 4W for base-stations and 100mW for remote devices, which are the limits set down by the US Federal Communications Commission. They can’t leak into adjacent channels, either, by ensuring signals neighbouring segments of the band are 55dB lower than the selected channel - Weightless uses single-carrier modulation (SCM) to minimise adjacent channel emission levels.

TV broadcasts have no such limits, so the M2M kit has to cope with interference from them too, in Weightless’ case by frequency hopping over 8MHz channels (6MHz in the US) at the frame rate and by temporarily removing consistently noisy frequencies from the list of those it can hop to at a given location.

What makes the 400-800MHz White Space band good for telly makes it good for M2M: signals go a long way - 100 miles is well within the range - and can easily reach deep into buildings. Neul’s test set-up blankets Cambridge in Weightless coverage at the cost of six base-stations. The number of cellular base-stations in the same area is an order of magnitude greater. Which means that Weightless could be rolled out for a tenth of the cost.

Tuned for the Internet of Things

It’s handy that M2M communications don’t need to be either as fast or as continuous as technologies designed for human communications. People want their call quality to be good and their films to download quickly, and that requires high data rates.

M2M applications are generally happy to go slow because, though sent regularly, their data payloads are very small, maybe a tenth of a bit every second. That allows Weightless to trade speed for range - reach is more important than getting the data very quickly, especially when you’re trying to keep the power consumption low.

Weightless makes use to Time-Division Duplex (TDD) transmission to allow applications to select the most appropriate balance between downlink and uplink traffic. Essentially, base-station and device transmit in their own time, not simultaneously. Transmissions are encrypted.

All good stuff, then, but can Neul and co. get it established as a standard, a de facto one if not a formally ratified specification - the Weightless SIG is already pitching the technology to standards body ETSI? They certainly hope so.

Neul says it has base-stations and sample client chipsets available now and expects to be in volume production in six months’ time. Ofcom may still be consulting spectrum stakeholders about its proposed White Space usage rules, but the FCC has set down its regulations, and Whitespace follows them.

It follows the proposed UK rules too. Backers of the standard believe the rest of the world will follow the lead of the Americans or the Brits with only minor variations, and the Weightless specification can evolve to cope with all but the most radically divergent of such differences.

Not that there will be any, the supporters say. Like Wi-Fi before it, they think Weightless will do as much to define regional regulations as to follow them.

Being open - the specification is available for use Weightless SIG members on a mix of royalty free and FRAND terms - Weightless has an accessibility that comparable proprietary technologies lack. There's little other activity for White Space M2M. The IEEE is working on extending Wi-Fi into White Space, but its specification, 802.11af, is still in development and even the standards body doesn't expect it to become a ratified standard before H2 2014.

All of which means standardised White Space-hosted M2M and internet of things connectivity using Weightltess could be in play by this time next year, with major roll-outs to follow. ®